1. Field of the Invention
This invention relates to semiconductor anisotropic resistor feed throughs and a temperature gradient zone melting process for making the same.
2. Description of the Prior Art
Heretofore anisotropic resistor feed throughs have usually been made by providing a wafer of high resistivity material with a plurality of apertures. Each aperture extends entirely through the wafer from one opposed surface to the other opposed surface. Each of these apertures is designed to receive a high electrical conductivity electrode which is sealed to the low conductivity wafer around the perimeter of the aperture. In cases where the anisotropic resistor feed through is desired as a barrier between different environments on the opposed faces of the wafer, these seals must not leak. Most seals are susceptible to electrochemical attack between the highly conductive electrode and the high resistivity wafer because of the difference in chemical deposition between these two bodies and thus are liable to leakage after long exposure times. Moreover, when the two materials composing the matrix wafer and the electrodes have different coefficients of thermal expansion, temperature fluctuations can result in stresses and strains that give rise to leakage. If the difference in the ambient pressures in the two different environments to which the opposed surfaces of the anisotropic resistor feed through is exposed is large, then the anisotropic resistor feed through must have considerable thickness in order that the resistor have the mechanical strength to withstand the pressure differential. For example, in a deep seal marine environment, an anisotropic resistor feed through may be exposed to both electrochemical attacks and high pressure differences. Finally, many anisotropic resistor feed throughs that are constructed of metal electrodes and cured organic resins are not resistive to either high temperatures or severe oxidizing environments.
An object of this invention is to provide a new and improved semicoductor anisotropic resistor feed through which corrects the deficiencies of the prior art devices.
Another object of this invention is to provide an anisotropic resistor feed through having an array of high conductivity channels of recrystallized semiconductor material in a thick wafer of high resistivity semiconductor material.
Another object of this invention is to provide a new and improved anisotropic resistor feed through including an array of high conductivity channels whose cross-sectional configuration may be round, square, triangular or diamond shape.
Another object of this invention is to provide a new and improved anisotropic resistor feed through whose chemical composition as far as electrochemical attack is concerned is the same in both the electrode and matrix region.
Another object of this invention is to provide a new and improved anisotropic resistor feed through having a wafer thickness as great as required to withstand large differences in pressure between the opposed faces of the wafer.
Another object of this invention is to provide a new and improved semiconductor anisotropic resistor feed through whose isolation between the high electrical conductivity electrode and low electrical conductivity matrix is a result not only of the difference in resistance between these two bodies but resulting from a blocking reversed bias P-N junction between the semiconductor electrode region and the semiconductor matrix region.
Another object of this invention is to provide a new and improved semiconductor anisotropic feed through whose high conductivity electrode and low conductivity matrix regions have the same coefficient of thermal expansion in order to minimize failure of the electrode as a result of stressing or the occurance of leaks because of temperature fluctuations.
Another object of this invention is to provide a new and improved semiconductor anisotropic resistor feed through which is resistant to severely oxidizing environments.
Another object of this invention is to provide a new and improved semiconductor anisotropic resistor feed through which is not destroyed by exposure to high temperatures.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.
In accordance with the teachings of this invention, there is provided an anisotropic resistor comprising a body of single crystal semiconductor material having two major opposed surfaces, a first selected level of resistivity and a first type conductivity. The semiconductor material is one selected from the group consisting of silicon, silicon carbide, gallium arsenide, germanium, a compound of a Group III and a Group V element, and a compound of a Group II and a Group VI element. At least one, or an array of, electrically conductive channel region is disposed in the body. Each channel region extends between, and terminates in, the two opposed major surfaces and has opposed end surfaces. Each end surface is coextensive with the respective major surface in which the end surface is disposed. The material of each of the channel regions is recrystallized semiconductor material of the body and has a second selected level of resistivity and a second type conductivity which may be the same as, or of opposite type conductivity to that of the body. The material of each of the channel regions has at least one metal impurity substantially uniformly distributed throughout the channel region, the concentration of which is sufficient to impart the second type conductivity and second selected level of resistivity thereto. The recrystallized material has a solid solubility of the impurity contained therein for the temperatures at which the process was practiced. When the second type conductivity is the same as the first conductivity, the resistance anisotropy of the feed or lead through will be derived solely from the resistivity difference between the material of the body and the material of the channel regions. When the second type conductivity is opposite to that of the first type conductivity, the resistance anisotropy of the feed or lead through will be derived both from the resistivity difference between the material of the body and that of the channel region and from the blocking characteristics of the P-N junction formed by the contiguous surfaces of the two materials.
Temperature gradient zone melting is utilized to produce the at least one channel region of the anisotropic resistor feed or lead through of this invention. A layer of metal which is the dopant material in itself, or includes a dopant material, is vapor deposited into one or more selectively etched depressions formed in a major surface of the body of semiconductor. The etched depressions are produced by a selective chemical etching and photolithography technique discussed in our following copending applications, all of the teachings therein being incorporated by reference thereto into this application: "Method of Making Deep Diode Devices", U.S. Pat. No. 3,901,736; "Deep Diode Device Production and Method", U.S. Pat. No. 3,910,801; "Deep Diode Devices and Method and Apparatus", Ser. No. 411,001, filed on Oct. 30, 1973, and now abandoned in favor of continuation application Ser. No. 552,154, filed on Feb. 24, 1975; "High Velocity Thermal Migration Method of Making Deep Diode Devices", U.S. Pat. No. 3,898,106; "Deep Diode Device Having Dislocation-Free P-N Junctions and Method", U.S. Pat. No. 3,902,925; and " The Stabilized Droplet Migration Method of Making Deep Diodes Having Uniform Electrical Properties", U.S. Pat. No. 3,899,361. The body or semiconductor wafer can have a &lt;111&gt;, a &lt;100&gt; or a &lt;110&gt; axial orientation. The etched depressions. have, respectively, triangular, square and diamond shapes for the &lt;111&gt;, &lt;100&gt; and &lt;110&gt; axial wafers which will produce channel regions having respectively triangular, square and diamond shaped cross-sections. In addition, etched linear depressions of finite length with any line direction orientation on the &lt;111&gt; axial wafer, with a &lt;011&gt; and/or a &lt;011&gt; line deduction on a &lt;100&gt; axial wafer and with a &lt;110&gt; line direction on a &lt;110&gt; axial wafer will produce channel regions with rectangular cross-sections with a high aspect ratio.
The metal deposited in the etched depressions is migrated through the body or wafer of semiconductor material of the first type from the bottom face to the top opposed face of the wafer to form the channel regions. When each channel region is of an opposite conductivity relative to the body, a P-N junction is formed which is a step junction. In particular, the migration of an array of aluminum-rich droplets through high resistivity N-type silicon will produce the P-N junctions and the desired configuration of channeled regions of a selective low level of resistivity and of P-type conductivity.